I have a similar problem in XPSP2. I run Mozilla Firefox as well as IE6. Niether comes up as quickly as they used to. I,m on BT Broadband 512Kbs and once they are open, have no problems.
As far as I can be sure, machine is as clean as a whistle from virus attacks, spyware, adware etc..
Hanging around is a bit frustrating but I can't find a solution.
Did you ever get your's fixed?
Obliged if you'll let me know.